Dynamic series voltage compensator and method thereof

ABSTRACT

A dynamic series voltage compensator ( 10 ) for compensating voltage dips in an ac electric power system is described. The compensator ( 10 ) has a controller ( 11 ) coupled to control a respective series injection inverter ( 12   a   , 12   b   , 12   c ). The controller ( 11 ) monitors a supply voltage with/on a respective phase. Inputs from monitoring the supply voltage are processed by the controller ( 11 ) to generate control signals. These control signals are based on comparing voltage magnitude of a present voltage cycle period with voltage magnitude of a preceding voltage cycle period of the supply voltage. When difference between the present and the preceding voltage cycle periods exceeds a predetermined value for a corresponding time period, and inverter control signal is provided to the respective series injection inverter ( 12   a   , 12   b   , 12   c ) to thereby provide a compensation voltage. The compensation voltage is injected directly to the respective conductor. The voltage magnitude of the compensation voltage is to compensate the supply voltage to a normal level based on the preceding voltage cycle period.

FIELD OF THE INVENTION

This invention relates to alternating current (ac) electric powersystems for providing ac electric power. In particular, this inventionrelates to series voltage compensators for compensating voltage dips insuch ac electric power systems and method thereof.

BACKGROUND

As is known in the art, an alternating current (ac) electric powersystem provides for generation, transmission and subsequent distributionof ac electric power to consumer locations. These consumer locations canbe, for example, residential homes, commercial premises and industrialbuildings or factories. Typically, an ac electric power system includes,among other things, conductors on which ac electric power is supplied tothe consumer locations. Examples of such conductors include undergroundcables and overhead lines.

Conventionally, voltage of ac electric power generated from a powersource is first stepped up for transmission and then stepped down fordistribution. Generally, distribution voltages are in the kilovolt (kV)range. Such distribution voltages are then stepped down to a consumervoltage level that is commonly at 400 Volts (V). In most ac electricpower systems, ac electric power is supplied to a consumer location viaa multiplicity of connections forming an ac power supply network.

In distributing ac electric power to consumer locations, faults mayoccur within an ac power supply network. These faults can be, forexample, an underground cable damaged by civil works or an overhead linefailure due to lightning strikes. Such faults adversely affect supplyvoltages to consumer locations and are commonly referred to as voltagedisturbances.

One type of voltage disturbance is known as a voltage dip. A voltage dipis a sudden and momentary reduction in a supply voltage from a normallevel. Generally, magnitude and duration of a voltage dip depends on thecauses of the voltage dip and also on control measures that areimplemented to restore the supply voltage to its normal level. Forexample, the duration of a voltage dip typically depends on, among otherthings, the time taken to identify the fault location causing thevoltage dip and for circuit breakers to trip and isolate the faultlocation.

Generally, the magnitude of a voltage dip is greater when nearer, inelectrical terms, to the fault location causing the voltage dip. Hence,the magnitude of the voltage dip is usually different at differentconsumer locations and may range from 10% to 80% of a supply voltage.Also, in ac electric power systems providing supply voltages in two ormore phases (polyphase), the magnitude of a voltage dip is generallydifferent in each of these phases. Often, a single-phase line-to-groundfault at a fault location can end up as a voltage dip on all phases at aconsumer location This is due to use of star-delta transformers that areknown to transfer at least some magnitude of a voltage dip in one phaseto the other phase(s).

One technique to compensate for a voltage dip is described in U.S. Pat.No. 5,329,222 issued to Gyugyi et al on Jul. 12^(th), 1994. This patentdescribes an apparatus and method for compensating utility linetransients with a series injection voltage. However, the use of athree-phase inverter and a transformer for coupling the three-phaseinverter into a high voltage distribution system results in the seriesinjection voltage on each phase being coupled to each other to someextent. Coupling the series voltage injection voltage as such is notappropriate because a voltage dip may be different on all three phasesand varying differently in time for each of these three phases.

Another technique to compensate for voltage dips is described in U.S.Pat. No. 5,883,796 issued to Cheng et al on Mar. 16^(th), 1999. Thispatent describes an apparatus and method for restoring voltage dipsusing a three-phase series injection inverter. Consequently, injectionvoltages provided by the apparatus and method of this patent has asimilar limitation as in U.S. Pat. No. 5,329,222 in that the injectionvoltages are again coupled to some extent.

Yet another technique to compensate for voltage dips in supply voltagesis with a current-to-voltage compensator. Current-to-voltagecompensators operate on the basis that most voltage disturbances are dueto single-phase line-to-ground faults in which the remaining phase(s)is(are) normal. By taking current from the normal phase(s) during asingle-phase voltage dip, and by means of a semiconductor inverterconverting this current into a series compensation voltage, the phasehaving the voltage dip can thus be compensated. Consequently,current-to-voltage compensators cannot adequately compensate voltagedips for all phases, particularly when all three phases have voltagedips.

In addition to the difficulty of compensating voltage dips in a singlephase for a polyphase ac electric power system, energy storage in theabove compensators is also a problem. This is because capacitors aretypically used to store energy to provide injection or compensationvoltages. Such capacitors can be expensive when a large energy storagecapacity is required so as to provide compensation voltages for voltagedips of long durations.

Furthermore, voltage compensators using series injection invertersprovide inverter voltages that are typically insufficient in magnitudeto compensate voltage dips in ac electric power distribution systems. Assuch, these inverter voltages have to be stepped up in magnitude usingstep-up transformers. Use of step-up transformers adds significantly tothe cost of conventional voltage compensators and this makes suchcompensators less desirable for general low-cost applications.

In addition to the above voltage compensators, uninterrupted powersupplies (UPSs) can also be used to compensate voltage dips on one ormore phases of an ac electric power system. However, UPSs are designedprimarily to compensate another type of voltage disturbance known as avoltage collapse. In a voltage collapse, supply voltages to consumerlocations are totally absent. Consequently, a UPS has to fully providethe supply voltages over the entire duration of the voltage collapse.This duration is typically much longer than that of voltage dips. Hence,a UPS requires energy storage that is substantially larger in capacitycompared to voltage compensators having series injection inverters.Furthermore, inverters of UPSs operate in a continuous high frequencyswitching mode even in the absence of any voltage disturbance. Attendantlosses during the continuous high frequency switching mode makes a UPSinefficient under normal supply voltage conditions.

Voltage dips can cause substantial financial losses especially whencommercial or industrial operations are affected. Hence, alleviatingvoltage dips in an ac electric power system is desirable. Thus, a needclearly exists for a series voltage compensator that addresses the aboveproblems in ac electric power systems to thereby provide supply voltagesthat are stable and reliable without incurring substantial additionalcosts.

SUMMARY

In accordance with one aspect of the invention, there is disclosed adynamic series voltage compensator for compensating voltage dips in analternating current electric power system providing at least one supplyvoltage, each of the at least one supply voltage being at a respectivephase, the dynamic series voltage compensator including:

means for independently monitoring each of the at least one supplyvoltage;

means for generating digital signals indicative of voltage magnitude ofthe each of the at least one supply voltage over a present voltage cycleperiod;

means for comparing the digital signals with stored data indicative ofvoltage magnitude of the each of the at least one supply voltage over apreceding voltage cycle period;

means for determining difference between the digital signals and thestored data at corresponding time periods within the present andpreceding voltage cycle periods;

and

means for controlling, when the difference exceeds a predetermined valuefor a corresponding time period, at least one series injection inverterto inject a compensation voltage directly to a respective conductor onwhich the each of the at least one supply voltage is supplied, thecompensation voltage having a magnitude to compensate the each of the atleast one supply voltage to a voltage magnitude of the preceding voltagecycle period immediately before a voltage dip at the corresponding timeperiod.

Generally, the generating means can include means for filtering thedigital signals.

Typically, the dynamic series voltage compensator can further includemeans for storing the digital signals.

More typically, the storing means can include means for locking thestored data.

Generally, the controlling means can include means for controlling theat least one series injection inverter to receive energy from at leastone energy storage device for the compensation voltage.

Typically, the controlling means can include means for controlling atleast one solid-state earthing switch, the at least one solid-stateearthing switch being to selectably connect an input of the at least oneseries injection inverter to a reference ground or to the each of the atleast one supply voltage.

Generally, the controlling means can include means for controlling atleast one pulse generator, the at least one pulse generator providingpulses synchronised to drive the at least one series injection inverterto provide two output pulses within one switching period.

Typically, the controlling means can include means for controlling atleast one solid-state bypass switch, the at least one solid-state bypassswitch connecting an input of the at least one series injection inverterto an output of the at least one series injection inverter.

In accordance with another aspect of the invention, there is disclosed amethod for compensating voltage dips in an alternating current electricpower system providing at least one supply voltage, each of the at leastone supply voltage being at a respective phase. The method including thesteps of:

independently monitoring each of the at least one supply voltage;

generating, in response to the independently monitoring step, digitalsignals indicative of voltage magnitude of the each of the at least onesupply voltage over a present voltage cycle period;

comparing the digital signals with stored data indicative of voltagemagnitude of the each of the at least one supply voltage over apreceding voltage cycle period;

determining difference between the digital signals and the stored dataat corresponding time periods within the present and preceding voltagecycle periods;

and

controlling, when the difference exceeds a predetermined value for acorresponding time period, at least one series injection inverter toinject a compensation voltage directly to a respective conductor onwhich the each of the at least one supply voltage is supplied, thecompensation voltage having a magnitude to compensate the each of the atleast one supply voltage to a voltage magnitude of the preceding voltagecycle period immediately before a voltage dip at the corresponding timeperiod.

Generally, the generating step can include the step of filtering thedigital signals.

Typically, the method can further include the step of storing thedigital signals.

More typically, the storing step includes the step of locking the storeddata.

Generally, the controlling step can include the step of controlling theat least one series injection inverter to receive energy from at leastone energy storage device for the compensation voltage.

Typically, the controlling step can include the step of controlling atleast one solid-state earthing switch, the at least one solid-stateearthing switch being to selectably connect an input of the at least oneseries injection inverter to a reference ground or to the each of the atleast one supply voltage.

Generally, the controlling step can include the step of controlling atleast one pulse generator, the at least one pulse generator beingproviding pulses synchronised to drive the at least one series injectioninverter to provide two output pulses within one switching period.

Typically, the controlling step can include the step of controlling atleast one solid-state bypass switch, the at least one bypass switchconnecting an input of the at least one series injection inverter to anoutput of the at least one series injection inverter.

In accordance with a further aspect of the invention, there is discloseda computer program product with a computer usable medium having acomputer readable program code means embodied therein for compensatingvoltage dips in an alternating current electric power system providingat least one supply voltage, each of the at least one supply voltagebeing at a respective phase. The computer program product including:

computer readable program code means for independently monitoring eachof the at least one supply voltage;

computer readable program code means for generating digital signalsindicative of voltage magnitude of the each of the at least one supplyvoltage over a present voltage cycle period;

computer readable program code means for comparing the digital signalswith stored data indicative of voltage magnitude of the each of the atleast one supply voltage over a preceding voltage cycle period;

computer readable program code means for determining difference betweenthe digital signals and the stored data at corresponding time periodswithin the present and preceding voltage cycle periods;

and

computer readable program code means for controlling, when thedifference exceeds a predetermined value for a corresponding timeperiod, at least one series injection inverter to inject a compensationvoltage directly to a respective conductor on which the each of the atleast one supply voltage is supplied, the compensation voltage having amagnitude to compensate the each of the at least one supply voltage to avoltage magnitude of the preceding voltage cycle period immediatelybefore a voltage dip at the corresponding time period.

Generally, the computer readable program code means for generating caninclude computer readable program code means for filtering the digitalsignals.

Typically, the computer program product can further include computerreadable program code means for storing the digital signals.

More typically, the computer readable program code means for storing caninclude computer readable program code means for locking the storeddata.

Generally, the computer readable program code means for controlling caninclude computer readable program code means for controlling the atleast one series injection inverter to receive energy from at least oneenergy storage device for the compensation voltage.

Typically, the computer readable program code means for controlling caninclude computer readable program code means for controlling at leastone solid-state earthing switch, the at least one solid-state earthingswitch being to selectably connect an input of the at least one seriesinjection inverter to a reference ground or to the each of the at leastone supply voltage.

Generally, the computer readable program code means for controlling caninclude computer readable program code means for controlling at leastone pulse generator providing pulses synchronised to drive the at leastone series injection inverter to provide two output pulses within oneswitching period.

Typically, the computer readable program code means for controlling caninclude computer readable program code means for controlling at leastone solid-state bypass switch, the at least one solid-state bypassswitch connecting an input of the at least one series injection inverterto an output of the at least one series injection inverter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described hereinafter with reference tothe drawings, in which:

FIG. 1 is a schematic block diagram illustrating a dynamic seriesvoltage compensator in accordance with a preferred embodiment of theinvention;

FIG. 2 is a flowchart of a method for compensating voltage dips usingthe dynamic series voltage compensator of FIG. 1;

FIG. 3 is a schematic diagram illustrating examples of a seriesinjection inverter and an energy storage device of the dynamic seriesvoltage compensator of FIG. 1;

FIG. 4 is a signal process flow of a controller of the dynamic seriesvoltage compensator of FIG. 1;

FIG. 5a and FIG. 5b illustrate a simplified scheme for generatinginverter control signals by the controller of FIG. 4;

FIG. 6a and FIG. 6b are schematic block diagrams illustrating tworespective alternate embodiments of the dynamic series voltagecompensator of FIG. 1; and

FIG. 7 is a block diagram of an example of a computer system capable ofimplementing the method of FIG. 2 with the dynamic voltage compensatorof FIG. 1.

DETAILED DESCRIPTION

A dynamic series voltage compensator, a method and a computer programproduct for compensating voltage dips in an electric power system aredescribed. In the following, numerous details are provided for a morethorough description. It shall be apparent to one skilled in the art,however, that the invention may be practised without such details. Inother instances, well-known details have not been described at length soas not to obscure the invention.

The advantages of the embodiments of the invention are manifold. Oneadvantage is that voltage dips of different phases of a supply voltageare independently compensated. Therefore, such voltage dips can becompensated with compensation voltages of different magnitudes for eachof the different phases. Consequently, the supply voltage is moreaccurately restored using the embodiments of the invention compared withconventional series voltage compensators or systems.

Another advantage of the embodiments of the invention is thatcompensation voltages are directly injected into conductors of anelectric power system without using a transformer. This significantlyreduces cost of the embodiments of the invention as compared toconventional series voltage compensator in which compensation voltagesare injected onto conductors via transformers.

A further advantage of the embodiments of the invention is that thecompensation voltages provided by the embodiments of the invention arereferenced to a normal level of the supply voltage and not to absolutevoltage references. Consequently, such compensation voltages compensatethe supply voltage according to actual supply voltage requirements ofconsumers rather than absolute voltage references that may not be asaccurate.

Yet a further advantage of at least one of the embodiments of theinvention is that a voltage collapse does not remove a ground returnpath for compensation voltages that are provided to replace supplyvoltages. This is because an earthing switch in one embodiment of theinvention provides the ground return path when the voltage collapseoccurs.

Referring now to FIG. 1, a schematic block diagram of a dynamic seriesvoltage compensator 10 for compensating voltage dips in an alternatingcurrent (ac) electric power system in accordance with a preferredembodiment of the invention is illustrated. The dynamic series voltagecompensator 10 includes a controller 11, at least one series injectioninverter 12 a, 12 b, 12 c and at least one energy storage device 13 a,13 b, 13 c respectively coupled to the series injection inverters 12 a,12 b, 12 c. The controller 11 couples to respective conductors 14 a, 14b, 14 c on which respective supply voltages V_(supply) _(—) _(A),V_(supply) _(—) _(B) and V_(supply) _(—) _(C) are supplied to respectiveloads via the series injection inverters 12 a, 12 b, 12 c. Each of thesesupply voltages V_(supply) _(—) _(A), V_(supply) _(—) _(B) andV_(supply) _(—) _(C) has a respective phase. Each of the seriesinjection inverters 12 a, 12 b, 12 c has, respectively, an input 15 a,15b,15 c and an output 16 a,16 b,16 c. Each of the supply voltagesV_(supply) _(—) _(A), V_(supply) _(—) _(B) and V_(supply) _(—) _(C) isrespectively provided from the output 16 a,16 b,16 c to consumerlocations (not shown) such as residential homes, commercial premises andindustrial buildings or factories. These consumer locations have voltageload requirements illustrated as V_(load) _(—) _(A), V_(load) _(—) _(B)and V_(load) _(—) _(C).

Also indicated for the series injection inverter 12 a is a control input17 a for receiving control signals from the controller 11 and energycouplings 18 a & 19 a coupled to receive energy from the energy storagedevice 13 a. To simplify labelling of FIG. 1, the control input 17 a andenergy couplings 18 a,19 a are not similarly labelled for the remainingtwo series injection inverters 12 b,12 c.

In order not to obscure the invention, operation of the dynamic seriesvoltage compensator 10 shall be described using the supply voltageV_(supply) _(—) _(A), the series injection inverter 12 a and elementsassociated herewith. The other series injection inverters 12 b,12 coperate similarly. Referring now to FIG. 2, a method 20 for compensatingvoltage dips in an ac electric power system using the dynamic seriesvoltage compensator 10 is illustrated with a flowchart.

The method 20 begins at step 21 at which the controller 11 independentlymonitors the supply voltage V_(supply) _(—) _(A). Step 21 processesinput tapped from the supply voltage V_(supply) _(—) _(A) conducted onthe conductor 14 a. Thereafter, the method 20 continues to step 22 atwhich the input is processed to generate digital signals. These digitalsignals are indicative of voltage magnitude of the supply voltageV_(supply) _(—) _(A) over a present voltage cycle period. Following step22, the controller 11 compares the digital signals with stored data atstep 23. The stored data is indicative of voltage magnitude of thesupply voltage V_(supply) _(—) _(A) over a preceding voltage cycleperiod.

Comparing these voltage magnitudes enables the controller 11 todetermine, at decision step 24, the difference between the digitalsignals and the stored data. This difference is computed atcorresponding time periods of the present and preceding voltage cycleperiods. When the difference exceeds a predetermined value for acorresponding time period, output from the decision step 24 is a “yes”.Consequently, the controller 11 then controls the series injectioninverter 12 a to inject a compensation voltage directly to the conductor14 a at step 25. The compensation voltage has a magnitude thatcompensates V_(supply) _(—) _(A) to a voltage magnitude of the precedingvoltage cycle period immediately before a voltage dip at thecorresponding time period. Compensating each of the other supplyvoltages V_(supply) _(—) _(B), V_(supply) _(—) _(C), to therebycompensate the supply voltages V_(supply) _(—) _(B), V_(supply) _(—)_(C) to a voltage magnitude of their respective preceding voltage cycleperiods preceding the voltage dip is also done independently whennecessary and based on the predetermined value. Otherwise, when thedifference is less than the predetermined value, output from thedecision step 24 is a “No”. With this “No”, the method 20 returns tostep 21 at which the controller 11 monitors another time period of thepresent voltage cycle period of the supply voltage V_(supply) _(—) _(A).

As the difference between the digital signals and the stored data isbased on the voltage magnitudes of the present and the preceding voltagecycle periods of the supply voltage V_(supply) _(—) _(A), magnitude ofthe compensation voltage can thus be determined to compensate the supplyvoltage V_(supply) _(—) _(A) to a voltage magnitude of the precedingvoltage cycle period at the corresponding time period. This magnitude isthus compensating a voltage dip that reduces the voltage magnitude ofthe present voltage cycle period from a normal level that was providedat that corresponding time period of the preceding voltage cycle periodimmediately before the voltage dip.

Injecting the compensation voltage directly to the conductor 14 a can beachieved by controlling the series injection inverter 12 a to receiveenergy from the energy storage device 13 a. Referring now to FIG. 3, aschematic block diagram illustrates examples of the series injectioninverter 12 a and the energy storage device 13 a.

The series injection inverter 12 a has a H-type bridge with fourswitching blocks 31 a,31 b,31 c,31 d. Each of these four switchingblocks 31 a,31 b,31 c,31 d has a solid-state switch coupled to a diode.Pulse width modulated (PWM) signals are used to drive the H-type bridgesuch that, after filtering output from the H-type bridge with inductor32 and capacitor 33, a compensation voltage is supplied across capacitor33. The inductor 32 and the capacitor 33 filter out high frequencycomponents of the PWM signals.

Power for the solid-state inverter is obtained from the energy storagedevice 13 a that includes a capacitor bank 34 and battery bank 35. Theenergy storage device 13 a also includes a bridge rectifier 36 and atransformer 37.

Control signals V_(G) from the controller 11 are selectively provided torespective gates of each of the switching blocks 31 a,31 b,31 c,31 d.These control signals V_(G) activate the series injection inverter 12 ato receive energy from the energy storage device 13 a The energy storagedevice 13 a is charged over a period of time by the transformer 37 andbridge rectifier 36.

The control signals V_(G) from the controller 11 are PWM signals. ThesePWM signals drive the H-type bridge such that the desired compensationvoltage is derived from the energy storage device 13 a. Generation ofthe PWM signals from the controller 11 requires processing of the inputstapped from the supply voltage V_(supply) _(—) _(A) that is conducted onthe conductor 14 a.

Referring now to FIG. 4, a signal process flow 40 of the controller 11is illustrated. The inputs are passed through an analog-to-digital (A/D)converter 41 from which digital signals that are indicative of voltagemagnitude of the supply voltage V_(supply) _(—) _(A) are derived. Thesedigital signals are provided to an active low pass filter 42. Input tothe circuit gate 43 can be from the A/D converter 41 or the active lowpass filter 42 depending on whether a voltage dip has been detected.Setting of the circuit gate 43 is explained below.

From the active low pass filter 42, a low-noise signal output, labelledas V_(present) (normal), is compared to the stored data that is storedin a dynamic look-up table 44. The dynamic look-up table 44 storesinstantaneous values of the supply voltage V_(supply) _(—) _(A) sampledat regular intervals within one cycle period. This cycle period is acontinuously moving time window of the supply voltage V_(supply) _(—)_(A). The stored data in the dynamic look-up table 44 is sequentiallyupdated to cover one cycle period of the supply voltage V_(supply) _(—)_(A).

The stored data from the dynamic look-up table 44, labelled asV_(preceding), and signal output from the circuit gate 43, labelled asV_(present), are provided to a difference detector 45. From thedifference detector 45, a difference signal (V_(difference)) isgenerated. V_(difference) is indicative of the difference in the voltagemagnitude between the present and the preceding voltage cycle periods ofthe supply voltage V_(supply) _(—) _(A) for a corresponding time period.

In the voltage dip detector 46, V_(difference) is checked against apredetermined value. When V_(difference) is greater than thepredetermined value, then a command signal (V_(lock) is sent back to thedynamic look-up table 44 to lock the stored data. Hence, when a voltagedip occurs, instantaneous values in the dynamic look-up table 44 of thepreceding voltage cycle period immediately before the voltage dip islocked. The stored data in the dynamic look-up table 44 at thecommencement of a voltage dip is therefore a replica of the voltagecycle period of the supply voltage V_(supply) _(—) _(A) immediatelybefore the voltage-dip. Locking the stored data enables voltage dipswith duration of more than one cycle period to be consistently comparedwith the preceding voltage cycle period immediately before the voltagedip. It is to be noted that the voltage dip can occur at any point of acycle period. A gate control signal (V_(Gate) _(—) ₁) is also sent tochange the circuit gate 43 from ‘normal’ to ‘dip’.

Operation of the circuit gate 43 depends on V_(Gate) _(—) ₁. Undernormal conditions with no voltage dip, V_(Gate) _(—) ₁ sets the circuitgate 43 to provide V_(present) (normal) based on output receiveddirectly from the active low pass filter 42. Otherwise, when a voltagedip is detected, V_(Gate) _(—) ₁ sets the circuit gate 43 to provideV_(present) (dip) based on output received directly from the A/Dconverter 41.

When V_(difference) is greater than the predetermined value, anothergate control signal (V_(Gate) _(—) ₂) changes a gate circuit 48 from anormal mode to a PWM mode. In the normal mode, output from the gatecircuit 48 is provided by a normal mode signal generator 49. In the PWMmode, V_(difference) is provided to a PWM signal generator 47 and outputfrom the gate circuit 48 is provided by the PWM signal generator 47.

In the PWM mode, the PWM signal generator 47 produces a PWM outputsignal that serves as an inverter control signal. This inverter controlsignal is provided as V_(G) _(—) _(A) to control the dynamic seriesinjection inverter 12 a. V_(G) _(—) _(A) controls the switching blocks31 a,31 b,31 c,31 d to provide a compensation voltage to compensate thevoltage dip.

The PWM output signal is generated when the PWM signal generator 47 isactivated in response to V_(difference) being greater than thepredetermined value. FIG. 5a and FIG. 5b illustrate a simplified schemefor generating inverter control signals.

FIG. 5a illustrates a compensation voltage, V_(comp), in the form of asine wave that may be required to be injected into the conductor 14 a.This sine wave is built up from a series of pulses generated at afrequency of; for example, 3.3 kHz by the switching blocks 31 a,31 b,31c,and 31 d. The inductor 32 and the capacitor 33 of FIG. 3 filter outharmonic components of these pulses to provide a compensation voltageV_(comp). The PWM signal generator 47 produces the pulses necessary forswitching the switching blocks 31 a,31 b,31 c,31 d via which thecompensation voltage V_(comp) is generated. Two sets of pulses arerequired for the switching blocks 31 a,31 b,31 c,31 d. One set of pulsescontrols switching block 31 a and the other set of pulses controlsswitching block 31 c.

The switching blocks 31 b and 31 d are controlled by inverted signalsgenerated by hard-wired circuitry operating on the inverter controlsignals V_(G) _(—) _(A) for switching blocks 31 a and 31 c. FIG. 5billustrates the sequence of pulses produced by the PWM signal generator47. The fundamental repetition rate at which the PWM pulse pattern iscalculated is 1.66 kHz to thereby provide a switching period (T_(s)) of0.6 ms. When the switching block 31 a is switched ‘on’, the switchingblock 31 d is also initially switched ‘on’ to thereby enable current tobe provided to a load. During the initial period when the switchingblock 31 d is switched ‘on’, the switching blocks 31 b and 31 c areswitched ‘off’. However, in the middle of the ‘on’ period of theswitching block 31 a, the switching block 31 c is switched ‘on’ and theswitching block 31 d ‘off’. This means that the current to the load isinterrupted for the duration that the switching block 31 c is switched‘on’. The result of this sequence is that the voltage across the loadconsists of two pulses within the switching period T_(s). As a result ofthis sequence, the switching frequency of the series injection inverter12 a has been effectively doubled from 1.66 kHz to 3.33 kHz. Switchinglosses in the switching blocks 31 a,31 b,31 c,31 d have therefore beeneffectively halved for this switching frequency of 3.33 kHz. A furtheradvantage of this frequency doubling is a faster response time for thedynamic series voltage compensator 10.

Twice within every switching period, the times T_(a) and T_(b) arecomputed by the controller 11 in order that the mean value of outputvoltage pulses of the dynamic series injection inverter 12 a are equalto the mean value of the compensation voltage required over a respectivehalf switching period, 0.5 T_(s). The mean value of the compensationvoltage, V_(comp), is related to the half switching period 0.5 T_(s) dcbus voltage, V_(dc), at node 38, and the time T_(b) by the followingrelationship:

V _(comp) =V _(dc)×(T _(b)/0.5T _(s))

or

T _(b)=(V _(comp) /V _(dc))×0.5T _(s)

giving

T _(a)=(0.5T _(s) -T _(b))/2if T _(a) T _(c)

The time T_(b) is therefore dependent not only on the value of V_(comp)required to restore the voltage dip, but also dc bus voltage V_(dc).This means that with this calculation, the value of V_(comp) iscorrected for dc bus variations. Then, knowing the value of T_(a) andT_(b), the firing time of the switching block 31 a is T_(a) and thefiring time of the switching block 31 c is (T_(a)+T_(b)). Similarly inthe next half switching period 0.5 T_(s), the turnoff time of theswitching block 31 a is (T_(a)+T_(b)) and the turn-off time of theswitching block 31 c is T_(a). The controller 11 carries out thesecalculations for each of the series injection inverters 12 a,12 b,12 cfor all three phases and also keeps track of the half switching period0.5 T_(s) in each of the three phases. Each new half switching period0.5 T_(s) for each phase is computed at approximately 0.3 ms.

Referring now to FIG. 6a, a schematic block diagram of an alternateembodiment 60 of the dynamic series voltage compensator 10. As acelectric power systems are also subjected to voltage collapses, thealternate embodiment 60 provides for respective solid-state earthingswitches. Only one solid-state earthing switch 61 a is indicated for thedynamic series voltage compensator 10 in order to simplify FIG. 6a Thissolid-state earthing switch 61 a selectably connects the input 15 a ofthe series injection inverter 12 a to a reference ground 62 or to thesupply voltage V_(supply) _(—) _(A). This solid-state earthing switch 61a is disposed on a part of the conductor 14 a that is on a supply sideof the supply voltage V_(supply) _(—) _(A) and prior to the input 15 a.The solid-state earthing switch 61 a has two switches 63,64. In normaloperations and voltage dip conditions, the controller 11 sets switch 63in a closed position and switch 64 in an open position, therebyconnecting the supply voltage V_(supply) _(—) _(A) to the seriesinjection inverter 12 a. When the controller 11 detects a voltagecollapse, a switch control signal is provided to a control input 65 toopen switch 63 and close switch 64 for the duration of the voltagecollapse.

Referring now to FIG. 6b, a schematic block diagram of another alternateembodiment 66 of the dynamic series voltage compensator 10. Thisalternate embodiment 66 provides for a solid-state bypass switch 67 thatis connected between the input 15 a and the output 16 a of the seriesinjection inverter 12 a. Under normal conditions with no voltage dips,the solid-state bypass switch 67 operates in the closed position. Inthis closed position, load current passes through the solid-state bypassswitch 67 and not the series voltage injection inverter 12 a Thisenables the alternate embodiment 66 to operate at a higher efficiencycompared to the dynamic series voltage compensator 10 without thesolid-state bypass switch 67. This is because the load current has lowerlosses when conducted through the solid-state bypass switch 67 than whenconducted through the series voltage injection inverter 12 a. When avoltage dip occurs on the ac electric power system, the controller 11detects the voltage dip and sends a first control signal, via a controlinput 68, to open the solid-state bypass switch 67. The controller 11also sends a second, and simultaneous, control signal to the seriesvoltage injection inverter 12 a, via the control input 17 a, to activatethe series injection inverter 12 a The solid-state bypass switch 67remains in an ‘off ’ position and the series voltage injection inverter12 a is in an operating condition for the duration of the voltage dip.When the voltage dip ceases, the controller 11 sends out simultaneouscontrol signals to the solid-state bypass switch 67 and the seriesvoltage injection inverter 12 a to thereby close the solid-state bypassswitch 67 and to set the series voltage injection inverter 12 a in anon-operating condition.

The controller 11 in the embodiments of the invention can be implementedusing a computer program product that includes, for example, a computersystem 70 as shown in FIG. 7. In particular, the controller 11 can beimplemented as software, or computer readable program code, executing onthe computer system 70.

The computer system 70 includes a computer 71, a video display 72, inputdevices 73, 74. A communication input/output (I/O) signal bus 75provides for inputs and outputs between the controller 11 and thedynamic series voltage compensator 10 and the conductor 14 a.

The computer 71 includes the controller 11, a memory 76 that may includerandom access memory (RAM) and read-only memory (ROM), input/output(I/O) interfaces 77, 78, a video interface 79, and one or more storagedevices generally represented by in FIG. 7 with a storage device 80. Thememory 76 can be used to store the digital signals and serve as thedynamic look-up table 44. When stored in the memory 76, the digitalssignals derived from the present voltage cycle period can overwrite thestored data of the preceding voltage cycle period by control signalsfrom the controller 11.

The video interface 79 is connected to the video display 72 and providesvideo signals from the computer 71 for display on the video display 72.User input to operate the computer 71 can be provided by one or more ofthe input devices 73, 74 via the I/O interfaces 78. For example, a userof the computer 71 can use a keyboard as I/O interface 73 and/or apointing device such as a mouse as I/O interface 74. The keyboard andthe mouse provide input to the computer 71. The storage device 80 canconsist of one or more of the following: a floppy disk, a hard diskdrive, a magneto-optical disk drive, CD-ROM, magnetic tape or any otherof a number of non-volatile storage devices well known to those skilledin the art Each of the elements in the computer system 71 is typicallyconnected to other devices via a bus 81 that in turn can consist ofdata, address, and control buses.

The method steps for compensating voltage dips in an ac electric powersystem using the dynamic series voltage compensator 10 is effected byinstructions in the software that are carried out by the computer system70. Again, the software may be implemented as one or more modules forimplementing the method steps. That is, the controller 11 can be a partof a computer readable program code that usually performs a particularfunction or related functions.

In particular, the software may be stored in a computer readable medium,including the storage device 80. The computer system 70 includes thecomputer readable medium having such software or program code recordedsuch that instructions of the software or the program code can becarried out. The use of the computer system 70 preferably effectsadvantageous apparatuses for compensating voltage dips in an ac electricpower system using the dynamic series voltage compensator 10 inaccordance with the embodiments of the invention.

The computer system 70 simply provides for illustrative purposes andother configurations can be employed without departing from the scopeand spirit of the invention. The foregoing is merely exemplary of thetypes of computers or computer systems with which the embodiments of theinvention may be practised. Typically, the processes of the embodimentsare resident as software or a computer readable program code recorded ona hard disk drive (generally depicted as the storage device 80) as thecomputer readable medium, and read and controlled using the controller11. Intermediate storage of the program code and media content data andany data fetched from the network may be accomplished using the memory76, possibly in concert with the storage device 80.

In some instances, the program may be supplied to the user encoded on aCD-ROM or a floppy disk (both generally depicted by the storage device80), or alternatively could be read by the user from the network via amodem device connected to the computer 71. Still further, the computersystem 70 can load the software from other computer readable media Thismay include magnetic tape, a ROM or integrated circuit, amagneto-optical disk, a radio or infra-red transmission channel betweenthe computer and another device, a computer readable card such as aPCMCIA card, and the Internet and Intranets including emailtransmissions and information recorded on Internet sites and the like.The foregoing is merely exemplary of relevant computer readable mediaOther computer readable media may be practised without departing fromthe scope and spirit of the invention.

The dynamic series voltage compensator 10 as described in the aboveembodiments of the invention advantageously overcomes or at leastalleviates the disadvantages of conventional series voltage compensatorfor compensating voltage dips in ac electric power systems.

In the foregoing description, a dynamic series voltage compensator, amethod and a computer program product for compensating voltage dips inan electric power system are described. Although two embodiments aredescribed, it shall be apparent to one skilled in the art in view ofthese embodiments that numerous changes and/or modifications can be madewithout departing from the scope and spirit of the invention.

What is claimed is:
 1. A dynamic series voltage compensator forcompensating voltage dips in an alternating current electric powersystem providing at least one supply voltage, each of said at least onesupply voltage being at a respective phase, said dynamic series voltagecompensator including: means for independently monitoring each of saidat least one supply voltage; means for generating digital signalsindicative of voltage magnitude of said each of said at least one supplyvoltage over a present voltage cycle period; means for comparing saiddigital signals with stored data indicative of voltage magnitude of saideach of said at least one supply voltage over a preceding voltage cycleperiod; means for determining difference between said digital signalsand said stored data at corresponding time periods within said presentand preceding voltage cycle periods; and means for controlling, whensaid difference exceeds a predetermined value for a corresponding timeperiod, at least one series injection inverter to inject a compensationvoltage directly to a respective conductor on which said each of said atleast one supply voltage is supplied, said compensation voltage having amagnitude to compensate said each of said at least one supply voltage toa voltage magnitude of said preceding voltage cycle period immediatelybefore a voltage dip at said corresponding time period.
 2. The dynamicseries voltage compensator as claimed in claim 1, wherein saidgenerating means includes means for filtering said digital signals. 3.The dynamic series voltage compensator as claimed in claim 1, andfurther including means for storing said digital signals.
 4. The dynamicseries voltage compensator as claimed in claim 3, wherein said storingmeans includes means for locking said stored data.
 5. The dynamic seriesvoltage compensator as claimed in claim 1, wherein said controllingmeans includes means for controlling said at least one series injectioninverter to receive energy from at least one energy storage device forsaid compensation voltage.
 6. The dynamic series voltage compensator asclaimed in claim 1, wherein said controlling means includes means forcontrolling at least one solid-state earthing switch, said at least onesolid-state earthing switch being to selectably connect an input of saidat least one series injection inverter to a reference ground or to saideach of said at least one supply voltage.
 7. The dynamic series voltagecompensator as claimed in claim 1, wherein said controlling meansincludes means for controlling at least one pulse generator, said atleast one pulse generator providing pulses synchronised to drive said atleast one series injection inverter to provide two output pulses withinone switching period.
 8. The dynamic series voltage compensator asclaimed in claim 1, wherein said controlling means includes means forcontrolling at least one solid-state bypass switch, said at least onesolid-state bypass switch-connecting an input of said at least oneseries injection inverter to an output of said at least one seriesinjection inverter.
 9. A method for compensating voltage dips in analternating current electric power system providing at least one supplyvoltage, each of said at least one supply voltage being at a respectivephase, said method including the steps of: independently monitoring eachof said at least one supply voltage; generating, in response to saidindependently monitoring step, digital signals indicative of voltagemagnitude of said each of said at least one supply voltage over apresent voltage cycle period; comparing said digital signals with storeddata indicative of voltage magnitude of said each of said at least onesupply voltage over a preceding voltage cycle period; determiningdifference between said digital signals and said stored data atcorresponding time periods within said present and preceding voltagecycle periods; and controlling, when said difference exceeds apredetermined value for a corresponding time period, at least one seriesinjection inverter to inject a compensation voltage directly to arespective conductor on which said each of said at least one supplyvoltage is supplied, said compensation voltage having a magnitude tocompensate said each of said at least one supply voltage to a voltagemagnitude of said preceding voltage cycle period immediately before avoltage dip at said corresponding time period.
 10. The method as claimedin claim 9, wherein said generating step includes the step of filteringsaid digital signals.
 11. The method as claimed in claim 9, and furtherincluding the step of storing said digital signals.
 12. The method asclaimed in claim 11, wherein said storing step includes the step oflocking said stored data.
 13. The method as claimed in claim 9, whereinsaid controlling step includes the step of controlling said at least oneseries injection inverter to receive energy from at least one energystorage device for said compensation voltage.
 14. The method as claimedin claim 9, wherein said controlling step includes the step ofcontrolling at least one solid-state earthing switch, said at least onesolid-state earthing switch being to selectably connect an input of saidat least one series injection inverter to a reference ground or to saideach of said at least one supply voltage.
 15. The method as claimed inclaim 9, wherein said controlling step includes the step of controllingat least one pulse generator, said at least one pulse generatorproviding pulses synchronised to drive said at least one seriesinjection inverter to provide two output pulses within one switchingperiod.
 16. The method as claimed in claim 9, wherein said controllingstep includes the step of controlling at least one solid-state bypassswitch, said at least one solid-state bypass switch connecting an inputof said at least one series injection inverter to an output of said atleast one series injection inverter.
 17. A computer program product witha computer usable medium having a computer readable program code meansembodied therein for compensating voltage dips in an alternating currentelectric power system providing at least one supply voltage, each ofsaid at least one supply voltage being at a respective phase, saidcomputer program product including: computer readable program code meansfor independently monitoring each of said at least one supply voltage;computer readable program code means for generating digital signalsindicative of voltage magnitude of said each of said at least one supplyvoltage over a present voltage cycle period; computer readable programcode means for comparing said digital signals with stored dataindicative of voltage magnitude of said each of said at least one supplyvoltage over a preceding voltage cycle period; computer readable programcode means for determining difference between said digital signals andsaid stored data at corresponding time periods within said present andpreceding voltage cycle periods; and computer readable program codemeans for controlling, when said difference exceeds a predeterminedvalue for a corresponding time period, at least one series injectioninverter to inject a compensation voltage directly to a respectiveconductor on which said each of said at least one supply voltage issupplied, said compensation voltage having a magnitude to compensatesaid each of said at least one supply voltage to a voltage magnitude ofsaid preceding voltage cycle period immediately before a voltage dip atsaid corresponding time period.
 18. The computer program product asclaimed in claim 17, wherein said computer readable program code meansfor generating includes computer readable program code means forfiltering said digital signals.
 19. The computer program product asclaimed in claim 17, and further including computer readable programcode means for storing said digital signals.
 20. The computer programproduct as claimed in claim 19, wherein said computer readable programcode means for storing includes computer readable program code means forlocking said stored data.
 21. The computer program product as claimed inclaim 17, wherein said computer readable program code means forcontrolling includes computer readable program code means forcontrolling said at least one series injection inverter to receiveenergy from at least one energy storage device for said compensationvoltage.
 22. The computer program product as claimed in claim 17,wherein said computer readable program code means for controllingincludes computer readable program code means for controlling at leastone solid-state earthing switch, said at least one solid-state earthingswitch being to selectably connect an input of said at least one seriesinjection inverter to a reference ground or to said each of said atleast one supply voltage.
 23. The computer program product as claimed inclaim 17, wherein said computer readable program code means forcontrolling includes computer readable program code means forcontrolling at least one pulse generator, said at least one pulsegenerator providing pulses synchronised to drive said at least oneseries injection inverter to provide two output pulses within oneswitching period.
 24. The computer program product as claimed in claim17, wherein said computer readable program code means for controllingincludes computer readable program code means for controlling at leastone solid-state bypass switch, said at least one solid-state bypassswitch connecting an input of said at least one series injectioninverter to an output of said at least said one series injectioninverter.